Electro-optic crystals and photoconductive dipole antennas have been widely used in terahertz (THz) time-domain spectroscopy and related imaging applications. For example, U.S. Pat. No. 5,952,818, issued to Zhang et al., discloses an electro-optical sensing apparatus and method for characterizing free-space electromagnetic radiation suitable for real-time, two-dimensional, far-infrared imaging applications. Pulsed terahertz electromagnetic radiation illuminates the electro-optical crystal, modulating the index of refraction via the Pockels effect. A femtosecond optical pulse probes the field-induced change in the index of refraction by passing through the crystal. To convert the field-induced ellipticity modulation in the probe pulse into an intensity modulation, the probe pulse is analyzed by a compensator and polarizer, and detected by a photodetector.
U.S. Pat. No. 5,789,750, issued to Nuss, discusses the use of a photoconductive dipole antenna structure usable as either a terahertz transmitter or a terahertz detector. U.S. Pat. No. 6,078,047, issued to Mittleman et al., discloses a method and apparatus for terahertz tomographic imaging, the apparatus including a photoconductive terahertz transmitter that generates terahertz radiation for illuminating a test object, and a separate photoconductive terahertz detector for detecting pulses reflected by the object. Measurement of the relative time delays of pulses reflected by the object are used to determine the positions of dielectric interfaces in the object.
In the standard apparatus used for THz time-domain spectroscopy as described in the above patents and elsewhere in the art, however, a separate transmitter and receiver are used for the emission and detection of the THz signal. Because detection is the reverse process of emission, the transmitter and the receiver can be identical devices. Despite advantages to be gained by using a single device as both a transmitter and a receiver (a “transceiver”), terahertz transceivers have not previously been known or used in the art, primarily because of perceived technical hurdles and inherent complexity, such as the difficulty of providing an acceptable signal-to-noise ratio.